This invention relates to suction boxes, and more particularly to a cover for a suction box having a water feeding unit near its leading edge.
Fourdrinier machines deposit paper pulp on a fast moving foraminous belt known as a Fourdrinier fabric and water is drawn out of the pulp through the fabric to set up the initial paper sheet. Such machines are provided with suction boxes positioned after a series of table rolls, or foils, and they function to remove water from the paper sheet by utilizing a vacuum pump to obtain a pressure difference between the top and bottom surfaces of the forming paper sheet. Water is then drawn through the Fourdrinier fabric into the suction box, and by virtue of the suction effect the paper and paper sheet are held tightly against the suction box surface over which the fabric is travelling.
Since the fabric may travel over the suction boxes at rather high speeds, the frictional engagement between the fabric and the suction box may cause severe wear of the fabric and substantially reduce its useful life. Also, suction box covers can become excessively worn, thus necessitating maintenance to preserve an even fabric supporting surface throughout their extent, since the wear often is irregular over the surface area, or in aggravated cases replacement of the complete covers becomes necessary. As another consequence, frictional drag on the fabric may result in substantial increased power consumption for driving the fabric around the Fourdrinier machine.
The amount of frictional drag and wear of a fabric varies considerably from one Fourdrinier machine to another. In some machines the paper sheet will contain more water than in other machines when moving over the suction boxes, and the water flow will provide a wet operation in which wear and frictional drag of the fabric are quite small. In other machines, the water flow out of the sheet is less and the machine operation is quite dry in the vicinity of the suction boxes. Then friction between fabric and suction box is a greater problem.
The degree of wetness, or dryness, can also vary between suction boxes of a single machine. The paper sheet progressively becomes drier as it proceeds over the boxes, and it is even possible to visually observe a dry line extending transversely across the paper sheet which is drier to one side than the other. Thus, fabric wear and frictional drag are factors that may present greater problems in some installations than others.
To circumvent these difficulties, suction box covers which are presently commonly formed of a high density polyethylene material have been subjected to "fly cutting"--the cutting of small curved grooves in the surface. These grooves might acquire moisture from the underside of the fabric which would provide lubrication between the fabric and the cover surface. Another prior construction has been the installation of shower sprays in advance of suction boxes to spray water against the fabric bottom to clean off small paper fines. Some of the applied water may be retained by the underside of the fabric for lubrication over the suction box cover, however, most of the water would be doctored off the fabric by the leading edge of the suction box. This construction has not been intended as a means of reducing friction between a fabric and a suction box. Thus, neither practice has accomplished a successful lubrication, and these wear problems continue to plague the papermaking industry.
Although these problems are currently encountered with Fourdrinier machines equipped with suction boxes having high density, polyethylene covers, the perplexing wear problems have early antecedents with metal box covers. In the patent issued to Shorey, et al., U.S. Pat. No. 1,216,861 on Feb. 20, 1917, a metal trough fastened to the leading edge of a metal suction box cover has a pipe through which water flows and is delivered to the underside of the fabric before it passes over the suction box cover. The water is introduced onto the fabric to lubricate it prior to travel over the suction box cover to ameliorate the wear problems discussed heretofore. The present invention lubricates the wire to overcome these problems, but the construction utilized is intended to improve upon the Shorey et al. arrangement to provide a construction that achieves a substantially uniform application of lubricant to the fabric across the suction box length, and that is, in a preferred form, specially useable in plastic covers.